Automobiles, trucks, tractors, earth-moving vehicles, vehicles, and many other different types of vehicles (hereinafter collectively referred to as automotive vehicles) frequently include an internal combustion engine for powering their movement across the earth's surface. An automotive vehicle also includes a drive train for transmitting energy produced by the internal combustion engine into movement of the wheels, drive tracks or similar means by which the vehicle is driven across the earth's surface. To effectively accommodate the power characteristics of the internal combustion engine to the load of the vehicle that it must drive at various speeds over varying terrain, an automotive vehicle's drive train usually includes one or more transmission. Each transmission in an automotive vehicle includes a transmission power input shaft that receives energy from the internal combustion engine's power output shaft, and a transmission power output shaft for transmitting the engines energy onto the means for driving the vehicle across the earth's surface. Each transmission in an automotive vehicle also includes sets of gears, each one of which, when selected for coupling the transmission's power input shaft to its power output shaft, provides a different speed ratio between the rotation rates, respectively, of the transmission's power input and power output shafts.
To facilitate selecting a particular gear ratio and for smoothly accelerating an automotive vehicle from a stationary start, its drive train usually includes a clutch located between the automotive vehicles internal combustion engine and its transmission(s). This clutch selectively couples the internal combustion engine's power output shaft to the transmission's power input shaft. In one position of the clutch, it completely decouples the engine's power output shaft from the transmission's power input shaft. In another position, the clutch of an automotive vehicle provides a tight coupling between the internal combustion engine's power output shaft and the transmission's power input shaft. In this tightly coupled state, the internal combustion engine's power output shaft and the transmission's power input shaft rotate at the same speed. However, most clutches for automotive vehicles operating in this tightly coupled state are capable of passing only some maximum amount of torque from the internal combustion engine to the transmission without slippage occurring in the clutch. If a torque greater than this maximum amount is supplied to the clutch in its tightly coupled state, slippage occurs within the clutch that allows the power output shaft of the internal combustion engine to rotate at a speed different from that of the transmission's power input shaft.
Between these two extremes of clutch operation, either of being decoupled or of being tightly coupled, the design of most clutches used in automotive vehicles permit progressively varying the tightness of coupling between the engine's power output shaft and the transmission's power input shaft. In intermediate states between these two extremes, the clutch will transmit an amount of torque to the transmission without slippage that is less than the maximum amount that it will transmit when tightly coupled. Controllably coupling differing amounts of torque from the internal combustion engine to the means for driving the vehicle across the earth's surface permits smoothly accelerating an automotive vehicle into motion. Controllably coupling different amounts of torque from the internal combustion engine to the means for driving the vehicle through the clutch is also useful, particularly for heavy industrial vehicles such as trucks, tractors and the like when shifting the transmission smoothly from a set of gears having one ratio to another set having a different ratio.
Historically, a driver of an automotive vehicle usually operated its clutch through a direct mechanical linkage between the clutch and a clutch pedal located in the vehicle's passenger compartment near the driver. In some instances, a closed hydraulic system for operating the clutch by pressure on the clutch pedal replaces the direct mechanical linkage. More recently, to provide automatic electronic control of gear ratio selection, particularly in automotive vehicle's that include a microprocessor, it has become desireable to control clutch operation by means of an electrical signal rather than by the driver pressing on a clutch pedal. While some designs for clutches are known that permit an electrical current to directly effect coupling and uncoupling of the clutch, such clutches generally consume, and must therefore also dissipate, a significant amount of electrical power. Thus, even with microprocessor controlled operation of an automotive vehicle's transmission, it still appears desirable to continue controlling clutch operation indirectly by converting a control electrical signal from the microprocessor into a more powerful mechanical driving force for directly operating a conventional clutch.
In pursuing this indirect electronic control of automotive vehicle clutches, some automotive vehicle manufacturers have chosen to employ electro-hydraulic transmissions having hydraulically operated clutches. In such electro-hydraulic transmissions, a hydraulic pump supplies pressurized hydraulic fluid for energizing a hydraulic actuator, for example a piston or a bellows, that directly operates the clutch. In one design for such a clutch, springs hold the clutch in its disengaged position and a carefully controlled pressure of the hydraulic fluid from the pump overcomes the springs' force to effect engagement of the clutch. When the hydraulic pressure is removed from this clutch, the springs once again move the clutch into its disengaged state. By using the spring pressure to effect clutch disengagement and hydraulic pressure to effect clutch engagement, the clutch inherently disconnects the engine from the transmission when the engine is not running to power the hydraulic fluid pump. Furthermore, this method of operating an electro-hydraulic clutch inherently avoids creating a hazardous condition if the hydraulic fluid pump fails. With such an electro-hydraulically operated clutch, smoothly accelerating the vehicle into motion and smoothly shifting transmission gear ratios require a hydraulic valve that controls the pressure of the hydraulic fluid supplied to the clutch precisely in response to changing values of the controlling electrical signal.
U.S. Pat. No. 4,996,195 entitled "Transmission Pressure Regulator" issued on Oct. 30, 1990 to Ralph P. McCabe ("the McCabe patent") and discloses a valve for controlling the pressure of a fluid medium that is adapted for use in a control system such as that of an automatic transmission of an automotive vehicle.
The valve disclosed in the McCabe patent includes a cylindrically shaped, elongated, hollow aperture means or cage. Formed through the wall of the cage toward one end is a first set of apertures or ports. This first set of ports receives a supply pressure of hydraulic fluid, apparently from a pump (not depicted or described in the text or drawings of the McCabe patent). A second set of apertures or ports also passes through the wall of the aperture means or cage. The second set of ports is displaced laterally from the first set of ports along the length of the cage and located near the middle of the length of the cage. The hydraulic fluid in the second set of ports has a control pressure and, apparently, is supplied to the automatic transmission (not depicted or described in the McCabe patent). A third set of apertures or ports is formed in the wall of the cage. The third set of ports is displaced laterally along the length of the cage from both the first and second sets of ports and is located near the end of the cage furthest from the first set. The hydraulic fluid in this third set of ports has a sump or tank pressure, and appears to return from the valve to a tank (not depicted or described in the McCabe patent).
The inner surface of the cage is formed in the shape of a right, circular cylinder and receives a snugly fitting main spool. The spool is much shorter than the cage and can, therefore, move laterally back and forth within the cage while remaining totally enclosed therein. A broad trough encircles the outer surface of the spool about its mid-section to establish a first chamber between the outer surface of the spool and the inner surface of the cage. The width of this trough along the length of the spool permits the first chamber to couple immediately adjacent pairs of sets of ports to each other while not simultaneously coupling all three sets of ports to each other. As depicted in FIGS. 1 and 2 of the McCabe patent, when the spool is fully displaced toward the right, the first chamber couples the second set of apertures, i.e., the clutch ports, to the third set of apertures, i.e., the tank ports. Alternatively, when the spool is fully displaced toward the left, the first chamber couples the first set of apertures, i.e., the pump ports, to the second set of apertures, i.e., the clutch ports. Thus, precisely controlled motion of the main spool laterally within the cage couples the set of clutch ports either to the set of pump ports ox to the set of tank ports, and, as described in the McCabe patent, can thereby control the hydraulic fluid pressure in the clutch ports.
As depicted in FIGS. 1 and 2 of the McCabe patent, the outer surface of the spool is also encircled by a narrow trough located near its left end. This narrow trough establishes a second chamber between the outer surface of the spool and the inner surface of the cage. The second chamber appears to be always open to a flow of hydraulic fluid from the pump through the pump ports through the wall of the cage.
Located in the interior of the spool disclosed in the McCabe patent is a hollow first internal passage. The formation of this passage in the spool establishes a cup-shaped cavity that is open toward the right end of the spool and closed at the spool's left end. A passage, formed through the wall of the spool, connects this cup-shaped cavity to the second chamber. From FIGS. 1 and 2 of the McCabe patent, it appears that the first internal passage in the spool always receives a flow of hydraulic fluid from the pump through the pump ports in the cage and the second chamber regardless of the lateral position of the spool along the length of the cage.
The spool disclosed in the McCabe patent also includes a second internal passage that pierces both the wall of the broad trough and the left end surface of the spool. This second internal passage couples the pressure of hydraulic fluid in the first chamber to a second cavity located at the left end of the spool between the spool and an end cap. The end cap closes the end of the cage to the left of the spool and seals the second cavity so that fluid may enter and leave it only through the second internal passage. Because the second cavity opens only into the second internal passage, the pressure within this second cavity always equals the pressure of fluid within the first chamber. The end cap also compresses a first coil spring between its inner surface and the left hand surface of the spool. In the absence of any other force on the spool, this first coil spring urges the spool toward the right end of the cage as depicted in FIGS. 1 and 2 of the McCabe patent.
An annularly shaped poppet valve plate is located immediately to the right of the spool as depicted in FIGS. 1 and 2 of the McCabe patent, and partially obscures the right hand end of the cylindrically shaped interior of the cage. The full pressure of hydraulic fluid applied by the pump to the pump ports forces hydraulic fluid through the pump ports in the wall of the cage, the second chamber, and the first internal passage in the spool to the side of the poppet plate immediately adjacent to the right hand end of the spool. A second coil spring is compressed between the spool and the poppet plate at the right end of the spool and, according to the text of the McCabe patent, applies a force to the spool that is smaller than that applied by the first coil spring at the left end of the spool.
Located to the right of the poppet plate is a movable armature that is surrounded by a solenoid coil. An electrical current flowing through the coil applies a magnetic force to the armature. In the valve depicted in FIG. 1 of the McCabe patent, this electromagnetic force on the armature urges it to move laterally toward the left which tends to close the opening in the center of the annularly shaped poppet valve.
According to the text of the McCabe patent, closure of the poppet valve increases the pressure of the hydraulic fluid at the right end of the spool adjacent to the poppet plate. With the spool urged to the right end of the cage by the first coil spring, an increase in hydraulic fluid pressure on the right end of the spool urges it to move laterally to the left away from the poppet plate. Movement of the spool to the left causes the first chamber to move laterally away from the tank ports toward the pump ports. Lateral movement of the first chamber over the pump ports permits hydraulic fluid to flow from the pump ports to the clutch ports thereby increasing the pressure of the hydraulic fluid in the clutch ports. Increased pressure of the hydraulic fluid in the clutch ports is coupled via the second internal passage to the second cavity thereby increasing the pressure of the hydraulic fluid in the second cavity at the left end of the spool. An increasing pressure in the second cavity urges the spool to halt its lateral movement to the left away from the poppet plate and urges it to begin moving back to the right toward the poppet plate. According to the text of the McCabe patent, "the spool . . . will move axially in relation to the poppet plate . . . until the sum of the forces on the spool . . . are in equilibrium." The text of the McCabe patent also states that the second coil spring compressed between the poppet plate and the spool acts to reduce lateral oscillation of the spool due to changes in the pressure of hydraulic fluid at opposite ends of the spool. Thus, according to the McCabe patent, the combination of the poppet valve at the right end of the spool with the second internal passage in the spool and the second cavity at the left end of the spool along with the second coil spring, precisely controls the movement of the main spool laterally within the cage to adjust the pressure in the clutch ports.
Based upon the preceding description of the operation of the valve depicted in FIG. 1 of the McCabe patent, that valve may be characterized as a normally closed valve that couples the clutch ports to the tank ports when no current flows through the coil. Conversely, the valve depicted in FIG. 2 of the McCabe patent includes a spring which biases the poppet valve closed, and a magnetic field generated by an electric current flowing through the toil urges the armature to move toward the right thereby opening the poppet valve. According to the text of the McCabe patent, the hydraulic pressure applied to the right end of the spool of the valve depicted in FIG. 2 when no current flows through the coil causes the spool to move to the left thereby causing the first chamber to couple the clutch ports to the pump ports. Thus the valve embodiment depicted in FIG. 2 of the McCabe patent may be characterized as a normally open valve that couples the clutch ports to the pump ports when no current flows through the coil.
The text of the McCabe patent appears to lack an explanation of how closing and opening of the poppet valve depicted in the drawings of the patent may increase or decrease the pressure of hydraulic fluid present at the right end of the spool adjacent to the annularly shaped poppet plate. Accordingly, it appears that the valve disclosed in the McCabe patent may be commercially impractical for its intended purpose of controlling the pressure of hydraulic fluid in an automatic transmission of an automotive vehicle.
U.S. Pat. No. 4,996,195 entitled "Pilot-Operated Valve With Load Pressure Feedback" issued on May 3, 1988 to Kenneth J. Stoss and Richard A Felland ("the Stoss et al. patent") discloses a pilot-operated electro-hydraulic valve adapted for use in controlling a transmission of an automotive vehicle. The valve disclosed in the stoss et al. patent includes an electromagnetically controlled pilot valve that controls the operation of the valve's main spool. A pilot feedback passage couples the pressure of hydraulic fluid in the load or clutch port of the valve to a feedback chamber at one end of the pilot valve. The Stoss et al. patent discloses that a pilot feedback passage coupling the clutch port to the feedback chamber preferably includes a filtered orifice. The Stoss et al. patent appears to omit an explanation of the function provided by the filtered orifice.
Neither the McCabe patent nor the Stoss et al. patent disclose or solve a problem that occurs in the operation of clutches in electro-hydraulic transmissions known as spiking. Spiking is a phenomenon that results from abruptly halting fluid flow through a hydraulic system. Fluid flowing through a hydraulic system has two types of energy. Those two different types of energy are potential energy and kinetic energy. Potential energy is energy that is present due to the pressure of hydraulic fluid. Kinetic energy is energy that is present due to the flow of fluid through the hydraulic system.
When a clutch, or any other hydraulically operated device that is moving in response to a flow of hydraulic fluid reaches the mechanical limit of its travel, the hydraulic fluid flow through the system stops abruptly. This abrupt stopping of hydraulic fluid flaw converts the fluid's kinetic energy into potential energy thereby producing a sudden and abnormal increase, or spike, in the pressure of the hydraulic fluid. Under appropriate circumstances, this pressure spike may be heard audibly as a disturbing or alarming noise, and the pressure increase may be so severe that it causes failure of the hydraulic system.